This invention relates to a process for expanding a foam styrene polymer sheet wherein the sheet is impregnated with aqueous composition.
U.S. Pat. No. 2,857,625 (Carlson, Jr.) discloses a method for preparing extruded styrene polymer foams of low density which includes contacting an extruded styrene polymer foam with liquid or vaporized water heated to at least 90.degree. C.
U.S. Pat. No. 2,797,443 (Carlson, Jr.) discloses a process of making low density foam polystyrene of small uniform cell size by blending a mixture of the polymer, pentane, and between 2.0 and 3% water, and immediately thereafter extruding the blend.
U.S. Pat. No. 3,631,133 (Battigelli) discloses a process for making low density polystyrene beads which includes step-wise expansion of the beads by alternately steaming and "conditioning" by passing warm air over a bead mass while the mass is "in silo".
U.S. Pat. No. 3,341,638 to Mandel discloses a method of forming foam polystyrene panels wherein a layer of pre-foamed expandable polystyrene particles is sprayed with a wetting agent dissolved in water to humidify the particles. Immediately thereafter the layer is expanded by heating in an electric field.
Modern Plastics, January 1965, at pages 171-178 includes an article by Skinner et al., "How polystyrene foam expands." Table I at page 173 purports to give permeability of thermoplastic films to gases. The article notes that there are "no published data on permeation through membranes as thin as those in expanded polystyrene."
British Pat. No. 854,586 relates to a method of extruding expandable polystyrene to obtain tubing with a minimum thickness, wherein the extruded foamed polystyrene may be subjected to hot water or steam post expansion.
Production of porous shaped articles from thermoplastic materials is described in U.S. Pat. Nos. 2,744,291 (Stastny et al.) and 2,787,809 (Stastny).
Numerous methods are known for expanding foam styrene polymer sheets wherein air is incorporated into the sheets, as by air permeation of extruded styrene polymer sheets. Expansion methods using permeation with air have not been entirely satisfactory in many commercial applications in that long periods, e.g., 72 hours, are required to effect air permeation to an extent effective for obtaining sufficient expansion upon subsequent heating.
Colombo, U.S. Pat. No. 3,823,047, discloses a method wherein a polystyrene foam web stored for at least 24 hours in air at atmospheric pressure undergoes increase in thickness by subsequent heating.
It has now been found by practice of the present invention that foam styrene polymer sheet can be expanded to form low density foam or cellular products by a method wherein the foam sheet is contacted with aqueous composition, e.g., water, preferably at a temperature below 90.degree. C. (192.degree. F.). Contact of the foam sheet with water may advantageously be maintained at a relatively low temperature and for a relatively short period of time to effect impregnation with little or no expansion. Highly expanded foam styrene polymer products having minimum apparent density may be formed by heating the impregnated foam sheet to the softening point of the styrene polymer. Typically, substantially less time is required for effective impregnation with aqueous composition relative to the time required to impregnate foam styrene polymer with air using heretofore known methods for making low density foam products.
As used herein "apparent density" means the mass per unit volume of a material, where the unit volume includes voids.
As used herein, the term "softening point" is the temperature at which there is an abrupt drop in the modulus of elasticity as the material is heated from its hard and stiff state; and, for most polymers, this point can be said to approximate the glass transition temperature or be somewhat less than the glass transition temperature. For polystyrene, the modulus of elasticity begins dropping abruptly at about 82.degree. C. (180.degree. F.) while the glass transition temperature is about 100.degree. C. (212.degree. F.).
The "softening range" is the temperature range which extends from the softening point to a temperature corresponding to the beginning of a low modulus region where the material begins to behave like rubber or where the molecules exhibit free flowing characteristics. The softening range of polystyrene is from about 180.degree. F. to about 230.degree. F.
The term "sheet" as used herein means an article having two surfaces larger in area than any other single surface of the article and includes but is not limited to thin sheets, i.e., having thickness less than 0.125 inch. The sheets may be tubular, flat, i.e., the planes in which each large surface lies are generally parallel each to the other, or of other shape.